CN1114486C - Homogeneous catalytic bed and method for transforming hydrocarbons into aromatic compounds with said bed - Google Patents

Abstract

The invention relates to a catalyst comprising at least one amorphous matrix, at least one noble metal, at least one additional metal M and at least one halogen, wherein C _Pt is the local concentration of the noble metal for a catalyst particle; C _M is the local concentration of the additional metal M; C _X is the local concentration of the halogen; where in the bed of catalyst particles the local dispersion of C _Pt /C _M or C _Pt /C _X values along the diameter of the particles is said to be homogeneous , which corresponds to at least 70% of the C _Pt /C _M or C _Pt /C _X values within the bed of catalyst particles with a maximum deviation of 30% from the local average ratio. The invention also relates to a method for converting hydrocarbons into aromatic compounds using said catalyst, for example for gasoline reforming and for the production of aromatic compounds.

Description

Uniform bed of catalyst and method for its use in converting hydrocarbons to aromatic compounds

The invention relates to a uniform bed and catalyst particles which improve bimetallic and bifunctional effects, reduce local compositional fluctuations of the catalyst particles, and greatly improve catalytic performance, especially in terms of activity and gasoline yield. Such a bed is said to be "micron-scale homogeneous". Such particles may even be referred to as "nanoscale homogeneous". The invention also relates to the use of the catalyst in a process for the conversion of hydrocarbons to aromatic compounds, such as gasoline reforming processes and processes for the production of aromatic compounds.

Catalysts for gasoline reforming and/or aromatics production are well known. They generally contain a matrix, at least one noble metal from the platinum group, at least one halogen and at least one promoter metal, also known as an additional metal.

As promoter metals, tin is used especially in regenerative processes and rhenium in fixed bed processes.

Catalysts used in gasoline reforming and/or production of aromatic compounds are bifunctional catalysts, having two functions that primarily constitute corrective performance: a hydrodehydrogenation function to dehydrogenate naphthenes and hydrogenate coke precursors, and a hydrodehydrogenation function to make Acid function for isomerization of naphthenes, paraffins and cyclization of long chain paraffins. The hydrodehydrogenation function can be provided by oxides such as molybdenum oxide _MoO3 , chromium oxide _Cr2O3 or gallium oxide _Ga2O3 , or by a metal selected from column _{10 (} Ni, Pd, Pt) supply. Metals, especially platinum, are known to be much more reactive to hydrodehydrogenation reactions than the oxide phase, and it is for this reason that metal catalysts have replaced supported oxide catalyst. However, metals, such as Ni, also exhibit hydrogenolysis activity, which is detrimental to the desired gasoline yield when reforming gasoline and/or producing aromatic compounds, while palladium and platinum have only low hydrogenolysis activity Just a little bit. The addition of a second metal, such as tin, can greatly reduce this hydrogenolysis activity, thereby improving the selectivity of the catalyst. Moreover, the addition of a second metal such as iridium or rhenium not only enhances the hydrogenation performance of platinum, but also promotes the hydrogenation of coke precursors, thereby improving the stability of the catalyst. These various reasons have contributed to the success of bimetallic catalysts over the first generation of monometallic catalysts. Recently, a trimetallic catalyst has been proposed, which not only maintains the high stability of the bimetallic catalyst, but also improves the selectivity of the catalyst to gasoline.

Selectivity can be increased in various ways. In the prior art, U.S. Patent US-A-5,128,300 recommends an extruded catalyst with uniform distribution of tin, whose local composition fluctuates almost within 25% of the average tin content, that is, 0.1-2% by weight of the catalyst .

We have found that it is possible to make The catalyst performance is significantly improved, which is the subject of the present invention. It is thus possible to homogenize the bimetallic effect of the noble metal with the additional metal M and/or the bifunctional effect of the noble metal-acid in the particle bed, thereby improving the overall performance of processes using such catalysts.

More precisely, the invention relates to a catalyst comprising at least one amorphous matrix, at least one noble metal, at least one additional metal M and at least one halogen, wherein for a catalyst particle C _Pt is the noble metal C _M is the local concentration of the additional metal M and C _X is the local concentration of the halogen. In such a catalyst with a uniform bed of catalyst particles, the local dispersion of C _Pt /C _M or C _Pt /C _X values is called By homogeneous is meant a value corresponding to at least 70% of the catalyst particle bed C _Pt /C _M or C _Pt /C _X with a maximum deviation of 30% from the local mean ratio.

Amorphous catalyst substrates are generally refractory oxides such as magnesia, titania or zirconia, alumina or silica, either alone or in combination. Preferred supports contain or are alumina.

For gasoline reforming reactions and/or reactions producing aromatic compounds, the preferred substrate is alumina, which advantageously has a specific surface of 50-600 m ² /g, preferably 150-400 m ² /g.

The catalyst also contains at least one noble metal selected from the platinum group (Pt, Pd, Rh, Ir), preferably platinum. The catalyst may advantageously contain a noble metal such as platinum and may also contain iridium.

The additional metal M is selected from tin, germanium, lead, gallium, indium, thallium, rhenium, manganese, chromium, molybdenum and tungsten. For the moving bed reforming gasoline process and/or the process of producing regenerated aromatic compounds, the preferred metal is tin, which is very favorable for combination with platinum (catalyst containing Pt, Sn), and more advantageous for the catalyst also contains tungsten (containing Pt, Sn and the catalyst of W).

In a fixed bed process, the preferred metal is rhenium; it is very advantageous in combination with platinum (catalysts containing Pt, Re); it is more advantageous that the catalyst also contains indium (catalysts containing Pt, Re, In); it can also contain tungsten (catalyst containing Pt, Re, W or Pt, Re, In, W).

Halogen is selected from fluorine, chlorine, bromine and iodine, preferably chlorine.

The catalyst generally contains 0.01-2% (weight) of noble metal, 0.1-15% (weight) of halogen and 0.005-10% (weight) of additional metal M. Preferably the catalyst also contains up to 2% of additional metal M, and very advantageously more than 0.1% of this metal. Under these preferred conditions, the catalyst will perform better due to optimized bimetallic effects.

It should also be noted that the catalyst used in the gasoline reforming and/or aromatic compound production process is preferably substantially free of alkali metals.

The catalyst used in the bed is pellets, extruded rods, trilobal particles or any other commonly used type of particles.

C _Pt is the local concentration of the noble metal (expressed in % (weight)) (the noble metal is not necessarily platinum), C _M is the local concentration of the additional metal M (expressed in % (weight)) and C _X is the local concentration of the halogen (expressed in % ( Heavy) means).

This concentration can also be expressed as % (atomic) since the relative fluctuations are the same.

The bulk composition of the catalyst can be determined by X-ray fluorescence with the powdered catalyst, or by atomic absorption spectrometry after the catalyst has been acid-etched.

In comparison with the overall composition of the catalyst, the micron-scale local composition of the catalyst can be determined by electron microprobe, and supplemented by STEM (scanning transmission electron microscope) if necessary. This determination can be done by determining the content of platinum and additional metal M in a certain area along the diameter of the catalyst particle of the order of a few cubic microns, called the measurement cell. This measurement enables the determination of the microscopic distribution of metals within the particles.

This analysis was performed using a JEOL JXA 8800 electron microprobe (preferred equipment) or, if desired, a CAMEBAX model microelectron beam instrument (Microbeam), equipped with a four-wavelength dispersive spectrometer for each set of analyses. The acquisition parameters are as follows: accelerating voltage 20kV, current 30nA, Pt Mα, Sn Lα, Cl Kα lines and counting time 20 seconds or 30 seconds, depending on the concentration level. These particles (shown as small bowls) can be coated with resin and polished to their diameter.

It should be noted that the designation "diameter" does not refer only to pellets or extruded shapes, but generally to the diameter of any particle shape; the term "diameter" is used to denote the representative length of the particle at which it is to be measured.

This determination is made on a representative sample of the bed or on a batch of catalyst from the catalyst bed. It is also contemplated that the analysis should be performed on the basis of at least 30 measurements each of at least 5 catalyst particles uniformly distributed across the diameter.

C _Pt represents the local concentration of the noble metal (expressed in % (weight)) (the precious metal is not necessarily platinum), C _M represents the local concentration of the additional metal M (expressed in % (weight)) and C _X is the local concentration of the halogen (expressed in % (weight) )express).

This concentration can also be expressed as % (atomic) since its relative fluctuations are the same.

From the determination of the local concentrations C _Pt , C _X and C _M (measured at a particular location corresponding to the diameter of the particle), the local C _pt /C _M and/or C _pt /C _X ratios can be calculated.

For each radial position, a local mean ratio [C _pt /C _M ] _m and/or [C _pt /C _X ] _m (corresponding to the mean value of the local ratios of the different particles) is calculated.

Thus, the absolute value of the difference between each locally determined C _pt /C _M ratio and the corresponding [C _pt /C _M ] _m mean local ratio can be determined. These values are called "local dispersion".

According to the invention, said dispersion is said to be homogeneous, referring to a maximum of at least 70%, preferably at least 80%, of the C _pt /C _M or C _pt /C _x values to this local mean ratio for a bed of catalyst particles The deviation is within 30%.

Thus, this means that a local dispersion of at least 70% of the particles corresponds to a confidence interval of more than 30%. Preferably, the criterion for the homogeneity of this local dispersion is reduced to 30%, preferably to 20%, advantageously to 15%, even 10%, or even 7% or 5% (that is to say that the value deviates from the maximum value within 20%, etc. wait).

Therefore, the variation of the amount of element M at any point in the catalyst is characterized by the control variable of platinum content in order to keep the Pt/M ratio within the optimal distribution range. This method can fully express the "bimetallic effect".

This bimetallic effect corresponds to the quality of the interaction between platinum and the metal M, whose role is to regulate the performance level of the catalyst.

The optimum C _Pt / _CM ratio is often on the side where the bimetallic effect is less pronounced (atomic ratio or % by weight), or beyond which the catalyst activity is reduced due to an excess of additional metal M. Such an optimum point is also observed for trimetallic catalysts between noble metals and metal M. In order to take full advantage of the benefits of the bimetallic effect due to the addition of one or several additional metals M, it is important that the locally determined C _Pt /C _M ratio for each catalyst particle vary as little as possible in this optimum value range .

Another very important parameter for the catalytic performance of catalysts, especially those used for gasoline reforming and/or aromatic compound production, is the amount of halogen (chlorine), especially the local concentration of halogen relative to the local concentration of noble metals . In this case, it represents the action of a bifunctional metal-acid.

Halogen (chlorine) determines the acidic function of the catalyst, which ensures the isomerization and cyclization of C ₆ -C ₁₁ paraffins. There is an optimum halogen content for each catalyst. The chlorine content is lower than this optimum content, and the catalyst will lack activity especially for the dehydrocyclization reaction of P ₇ -P ₉ paraffins. However, when the chlorine content exceeds this optimal content, the catalyst exhibits too high cracking activity, resulting in the generation of a large amount of C ₃ -C ₄ fuel gas, and thus reduces the yield of gasoline. The optimal chlorine concentration depends on the nature of the support, its specific surface area and its structure. The chlorine concentration is usually close to 1.0% by weight for commercial catalysts, but it can be significantly higher or lower than this value for some individual supports, or in the case of supports with doping elements such as silicon present.

This makes the local C _Pt /C _X concentration ratio significantly different from the local average ratio, resulting in moderate catalytic performance.

In general, the local C _Pt /C _X ratio or C _Pt / _CM ratio is constant along the diameter of the catalyst particle. The C _Pt /C _M distribution as a function of diameter is therefore also a "flat distribution", like the C _Pt , C _M or C _X distributions with diameter (depending on the case). The noble metal and/or the metal M and/or the halogen is homogeneously distributed in the grain.

For a given particle (preferably, a pellet), it is possible to determine separately the C _Pt /C _X ratio and the [C _Pt /C _M ] _p and/or C _pt /C _X ] _p average ratio for each locally determined intragranular the absolute value of the difference. These values are referred to as "radial dispersion within the particle".

Dispersion according to the invention is said to be homogeneous over each particle, meaning at least 70%, preferably at least 80%, of values with a maximum deviation of 30% from the intragranular mean.

Preferably this radial dispersion is reduced to 30%, preferably to 20%, advantageously to 15% or even 10%, or even 7% or 5%.

Following the same method as before, it can thus be said that for at least 70% of the particles the radial dispersion corresponds to a confidence interval higher than 30%.

For a given catalyst batch (e.g., for good representation, at least 5 particles with at least 30 measurements per particle), it is possible to determine the C _Pt /C _M or C _pt /C separately for each local determination The absolute value of the difference between the ratio of _X and the overall mean ratio of [C _Pt /C _M ] _L or [C _pt /C _x ] _L for the lot, respectively. These values are called "overall dispersion".

A dispersion according to the invention is said to be homogeneous, meaning at least 70%, preferably at least 80%, of values with a maximum deviation of 30% from the batch average (overall average ratio).

Preferably this overall dispersion is reduced to 30%, preferably to 20%, advantageously to 15% or even 10%, or even 7% or 5%.

Following the same method as before, it can thus be said that this overall dispersion corresponds to a confidence interval higher than 30% for at least 70% of the particles.

It is also of interest to prepare catalysts with different core and peripheral concentrations of C _Pt , C _M or C _X . These catalysts have a "bowl" or "arch" distribution pattern. These catalysts having a bowl or domed C _M or C _Pt distribution are of interest in certain applications where the effect of the diffusion rate of reactants or products within the catalyst is to be exploited.

In this case, the value of the local mean ratio [C _Pt /C _M ] _m varies as a function of particle size. This change can basically follow the law of the parabolic curve.

Another type of distribution is that of a "surface layer", where the precious metal and/or metal M is distributed on the surface.

Generally, the concentration ratio of C _pt , C _M or C _X between the core and the periphery of the catalyst particle can vary from 0.1 to 3.

In this preferred variant, the catalyst contains at least one metal M and a noble metal, preferably platinum, distributed homogeneously within the catalyst particles.

Another possibility is that the catalyst contains at least one metal M distributed homogeneously throughout the catalyst, and the noble metal in a bowl-shaped distribution. In another variation, at least one metal M is uniformly distributed throughout the catalyst, while the noble metal is distributed in a "surface layer".

The metal M is advantageously tin in the above case. Preferably, both platinum and tin are distributed in a bowl shape.

Accompanying drawing 1 to 4 illustrate the present invention and prior art:

Fig. 1A and Fig. 1 B represent bowl shape and arch distribution (not appended to embodiment);

Fig. 2 is equivalent to prior art;

3A, 3B, 4 and 5 correspond to the present invention described in the following examples.

General examples described in this patent are shown in Figures 1A, 1B, 3A and 3B; for a bed of catalyst particles at least 70% of the C _Pt /C _M or C _pt , C _X values deviate from the corresponding local average ratio along the particle diameter to a maximum of 30%.

The _CPt / _CM value can be described as a straight line (equivalent to catalysts B and C according to embodiments of the present invention as shown in Figure 3) or a hyperbola (as shown in Figure 1 "bowl" or "arch" distribution).

Very preferably, the catalyst contains at least one metal M in a homogeneous distribution throughout the catalyst, and the noble metal also in a homogeneous distribution across the catalyst particles.

In one technique of the present invention, the catalyst is obtained by impregnating an organic solution of at least one said metal M compound, the volume of which solution preferably equals or exceeds the hold-up volume of the support. Metal M is introduced in the form of at least one organic compound selected from metal M and hydrocarbyl complexes, such as selected from alkyl, cycloalkyl, aryl, alkaryl and aralkyl metals. After several hours of contact to separate the solids and the impregnating liquid, the product is then dried. This operation is usually carried out at temperatures between 300 and 600°C, preferably in air flow for several hours. The resulting solid is then impregnated with an aqueous or organic solution of at least one compound of a Group VIII metal, the volume of which preferably exceeds or is equal to the hold-up volume of the support. After remaining in contact for several hours, the resulting product is dried and calcined in air, preferably in a stream of air, at a temperature between 300°C and 600°C for several hours.

According to another method of the present invention, tin can be introduced during the synthesis of alumina using sol-type techniques (co-precipitation). As an example, a mixed tin alumina gel is obtained by hydrolyzing an organic solution of Sn(OR) ₄ and Al(OR') ₄ in a solvent such as ROH or R'OH. R and R' may represent methyl, ethyl, isopropyl, n-propyl, or butyl groups or heavier groups such as n-hexyl. Alcoholic solvents must be strictly dehydrated before being introduced into tin and aluminum alcoholates. Hydrolysis can be achieved by adding water to the mixture or adding anhydrous carboxylic acid followed by esterification with gradual heating (solvolysis). The second technique, since it allows the uniform and simultaneous formation of water in the mixture, generally results in a more uniform _{Al2O3 - SnOx} mixed oxide. The reactivity (hydrolysis) of tin alcoholates to water is generally higher than that of aluminum alcoholates, however, it decreases with the increase of the alkyl chain R. Thus, the molecular weight of the R and R' groups can be chosen to match the corresponding aluminum and tin alcoholates. This can further improve the uniformity of metal distribution in the resulting hybrid gel. It is also possible to co-precipitate tin and aluminum in an aqueous solution, for example, dissolve SnCl ₂ and AlCl ₃ in a solution acidified with hydrochloric acid, and then inject the acid solution in the form of droplets (spray, atomization) to a pH range of 6-9 in the aqueous solution.

These metals can be introduced by any method known to those skilled in the art. The additional metal M can be introduced at any stage of catalyst production, for example during the synthesis of alumina by the sol method, or during the shaping of the catalyst (rod extrusion, oil drop formation, or any known method).

According to the present invention, the above-mentioned catalysts can be used in the process of reforming gasoline and producing aromatic compounds. Reforming processes increase the octane number of gasoline fractions derived from the atmospheric distillation of crude oil and/or other refining processes. The production process of aromatic compounds provides the basic raw materials (benzene, toluene, xylenes) for the petrochemical industry. These processes are particularly important because they are used to produce large quantities of hydrogen, which is essential for refineries to complete hydrogenation and hydrotreating processes. These two processes are different from each other in the selection of operating conditions and in the composition of the feed, which is a fact well known to those skilled in the art.

In general, typical feedstocks processed by these processes are paraffinic, naphthenic and aromatic hydrocarbons having 5-12 carbon atoms per molecule. This feed is limited in particular by its density and weight composition. This feed is contacted with the catalyst of the invention at a temperature in the range of 400°C to 700°C. The feed mass flow rate treated by a unit mass of catalyst may be from 0.1 to 10 kg/kg/h. The operating pressure can be between normal pressure and 4MPa. Part of the generated hydrogen is circulated at a molar circulation ratio ranging from 0.1 to 10. This ratio is the molar ratio of recycle hydrogen flow rate to feed flow rate.

The following examples illustrate the invention without any limitation on the scope.

Embodiment (comparison) 1

The reference catalyst or Catalyst A was a bimetallic Pt-Sn catalyst prepared by known techniques from _SnCl2 comprising 0.25% by weight platinum and 0.14% by weight tin and 1.2% by weight chlorine. The carrier is γ-alumina with a specific surface area of 210m ² /g. Take 500 ml of hydrochloric acid aqueous solution and tin tetrachloride containing 0.14 g of tin, and add them to 100 g of alumina carrier. Keep in contact for 3 hours and remove. The solid was then contacted with 500 ml of hexachloroplatinic acid aqueous solution containing 0.25 g of platinum, kept in contact for 3 hours, dried at 120° C. for 1 hour, and then calcined at 500° C. for 2 hours.

Embodiment 2 (according to the present invention)

Catalyst B, having the same composition, was prepared by impregnation with an organometallic tin complex. 100 g of an alumina support was brought into contact with 60 ml of a solution of tetrabutyltin (Sn(Bu) ₄ ) in n-heptane containing 0.14 g of tin. After 3 hours of reaction at ambient temperature, the solid was dried at 120°C for 1 hour, followed by calcination at 500°C for 2 hours. A further 100 g of this solid was brought into contact with 500 ml of hydrochloric acid and an aqueous solution of hexachloroplatinic acid containing 0.25 g of platinum. After keeping it in contact for 3 hours, it was dried at 120°C for 1 hour, and then fired at 500°C for 2 hours.

Embodiment 3 (according to the present invention)

Catalyst C is prepared by co-precipitating aluminum and tin in an aqueous solution and then uniformly depositing platinum. It contains 0.25% by weight platinum, 0.14% by weight tin and 1.2% by weight chlorine. Mixed Al ₂ O ₃ -SnO ₂ .mH ₂ O hydroxide is prepared by co-precipitating tin chloride and aluminum chloride solution with NH ₄ NO ₃ as precipitant at pH 8. The precipitate was washed with distilled water and dried at 120°C for 12 hours. It was then calcined at 530°C for 2 hours in air containing 500 ppm water. Platinum was then incorporated into the solid by contacting 100 g of the solid with 500 ml of a toluene solution containing 0.25 g of platinum bisacetylacetonate. Contact was maintained for another 3 hours, dried at 120°C for 1 hour, and calcined at 500°C for 2 hours. The solid was impregnated with an aqueous solution of hydrochloric acid to adjust the chlorine content of the catalyst to 1.2%. The resulting solid was then dried and calcined at 500°C for 2 hours.

Example 4

The local platinum and tin concentrations of the three catalysts A, B and C were determined by electron microprobe technique. Figure 2 (for Catalyst A), Figure 3A (for Catalyst B) and Figure 3B (for Catalyst C) list the local dispersion ratio C _Pt /C _Sn around the local average ratio. For the catalyst, only 49% of the points were within the confidence interval (Figure 2). Catalysts B and C according to the invention exhibit very narrow point spreads, with 8% and 14% of the local C _Pt / _CSn ratios, respectively, outside the confidence zone.

Example 5

FIG. 4 illustrates the evolution of the local C _Pt / _CCl concentration ratio along the diameter of the catalyst particle B. FIG. It can be seen that 9% of the points are outside the confidence zone.

Example 6

Catalyst D contained 0.3% by weight platinum and 0.32% by weight tin and 1% by weight chlorine and was prepared by impregnating an organometallic complex. A quantity of 100 g of the alumina support was brought into contact with 60 ml of a solution of tributyltin Sn(Bu) ₄ in n-heptane containing 0.32 g of tin. After 3 hours of reaction at ambient temperature, the solid was dried at 120°C for 1 hour and calcined at 500°C for 2 hours.

100 g of this solid was then brought into contact with 500 ml of a solution of platinum bisacetylacetonate in toluene containing 0.3 g of platinum to introduce the platinum. Keep in contact for 3 hours, dry at 120°C for 1 hour, and then bake at 500°C for 2 hours. The solid was impregnated with 600 ml of an aqueous hydrochloric acid solution, introducing 1% by weight of chlorine (relative to the catalyst). After keeping it in contact for 3 hours, it was dried at 120°C for 1 hour, and then fired at 500°C for 2 hours.

The local concentrations of platinum and tin were measured with JEOL electronic microprobes, counting 200 points on 5 small balls, and the counting time was 40 seconds.

The local _CPt / _CM dispersion (atomic %) is presented in Figure 5 with a deviation of 9% for a value of 70%.

As for the overall dispersion, 70% of the values deviate within 11%. For each of the 5 pellets, for a radial dispersion value of 70%, the corresponding deviations are 9, 10, 11, 11, 12%, respectively.

Example 7

Catalyst samples A, B, C, and D, the preparations of which were described above, were tested by converting a feed with the following characteristics:

Density, 20℃ 0.753kg/ ^dm3

Research Octane Number ～60

Paraffin content 49.4% (vol)

Naphthene content 35.1% (vol)

 Aromatic hydrocarbon content

This transformation is carried out in the presence of hydrogen under the following conditions:

Temperature 490℃

Pressure 0.3MPa

Feeding rate 2.0kg/kg. Catalyst

The catalyst was activated at elevated temperature in hydrogen for 2 hours prior to injection of the feed. The properties obtained after catalyst operation for 24 hours are listed in the table below: sample Reformed product yield % (weight) Research Octane Number Aromatics yield % (weight) C ^4- yield % (weight) A 89.2 101.2 70.3 7.0 B 90.7 103.7 72.6 5.3 C 90.5 103.5 72.8 5.6 D. 90.9 103.7 73.3 5.1

Catalysts B, C and D have significantly better catalytic performance than Catalyst A, both in terms of the amount of reformate produced and the octane number of the reformate.

Claims (22)

1. A catalyst comprising one or several amorphous substrates, one or several noble metals, one or several additional metals M and one or several halogens, wherein for a catalyst particle, said The amorphous matrix is a refractory oxide selected from the group consisting of magnesia, titania, zirconia, alumina, silica, alone or in combination, C _Pt is the local concentration of noble metal, in the range of 0.01-2% by weight; C _M is the local concentration of the additional metal M, in the range of 0.1-2% by weight; C _X is the local concentration of halogen, in the range of 0.1-15% by weight; The catalyst is in the form of a homogeneous bed of catalyst particles where the local dispersion of C _Pt /C _M or C _Pt /C _X values is said to be homogeneous, which corresponds to 70% or more of the C _Pt /C within the bed of catalyst particles The maximum deviation of the C _M or C _Pt /C _X value from the local average ratio is 30%. 2. A catalyst according to claim 1, wherein the overall dispersion of C _Pt /C _M or C _Pt /C _X values is said to be uniform for a batch of particles, which corresponds to 70% or more of The maximum deviation of the C _Pt /C _M or C _Pt /C _X value from the overall mean ratio is 30%. 3. A catalyst according to claim 1, wherein the radial dispersion of the C _Pt /C _M or C _Pt /C _X values is said to be uniform for a catalyst particle, which corresponds to 70% or more The C _Pt /C _M or C _Pt /C _X value deviates from the particle average ratio by a maximum of 30%. 4. Catalyst according to claim 1, characterized in that the noble metal is platinum. 5. Catalyst according to claim 1, characterized in that the halogen is chlorine. 6. Catalyst according to claim 1, characterized in that the additional metal M is selected from tin, germanium, lead, gallium, indium, thallium, rhenium, manganese, chromium, molybdenum and tungsten. 7. The catalyst according to claim 1, characterized in that it is selected from catalysts containing Pt, Re; catalysts containing Pt, Re and In; catalysts containing Pt, Sn; catalysts containing Pt, Re, W; , Re, In, W catalysts; and catalysts containing Pt, Sn, W. 8. A catalyst according to claim 1, characterized in that the ratio between the concentration of _CPt , _CM or _Cx at the catalyst core and the concentration of _CPt , _CM or _Cx at the periphery of the catalyst is 0.1 to 3, respectively. 9. Catalyst according to claim 1, characterized in that one or more additional metals M are homogeneously distributed in the catalyst and the noble metal is also homogeneously distributed in the catalyst particles. 10. Catalyst according to claim 1, characterized in that one or more additional metals M are homogeneously distributed in the catalyst, the noble metal being distributed in a "bowl shape". 11. Catalyst according to claim 1, characterized in that one or more additional metals M are distributed homogeneously in the catalyst, the noble metal being distributed in a "skin". 12. Catalyst according to claim 1, characterized in that the metal M is tin. 13. Catalyst according to claim 1, characterized in that it contains platinum and tin in a bowl-shaped distribution. 14. Catalyst according to claim 1, characterized in that it comprises platinum and iridium as noble metals. 15. Catalyst according to claim 1, characterized in that the value deviates by a maximum of 20%. 16. Catalyst according to claim 1, characterized in that the value deviates from 0 to 15%. 17. Catalyst according to claim 1, characterized in that the value deviates from 0 to 10%. 18. Catalyst according to claim 1, characterized in that the value deviates from 0 to 7%. 19. Catalyst according to claim 1, characterized in that the value deviates from 0 to 5%. 20. A process for converting hydrocarbons to aromatic compounds using a catalyst according to claim 1. 21. A method according to claim 20 for gasoline reforming. 22. A process according to claim 20 for the production of aromatic compounds.

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